Antibiotics are chemical substances having the capacity, in a dilute solution, to kill or inhibit growth of microorganisms. Antibiotics that are sufficiently nontoxic to the host are used as chemotherapeutic agents to treat infectious diseases of humans, animals, and plants. The term was originally restricted to substances produced by microorganisms, but has been extended to include synthetic and semi-synthetic compounds of similar chemical activity.
Antibiotic drugs are commonly classified based on their chemical structure, mechanism of action, or spectrum of activity. Most antibiotics target bacterial functions or growth processes (Calderon C. B., et al., Antimicrobial Susceptibility Testing Protocols. 2007, CRC Press. Taylor & Frances group) that are not in common with eukaryotic organisms. The major families of antibiotic drugs can be classified as follows: Penicillins and Cephalosporins are antibiotic drugs that target the bacterial cell wall. Polymyxins target the bacterial cell membrane. Rifamycins, Lipiarmycins, Quinolones, and Sulfonamides interfere with essential bacterial enzymes. Macrolides, Lincosamides and Tetracyclines target protein synthesis (Finberg R. W., et al., Clin. Infect. Dis. 2004, 39 (9): 1314-20). Further categorization is based on their target specificity, and thus, narrow-spectrum antibacterial antibiotics target specific types of bacteria, whereas broad-spectrum antibiotics affect a wide range of bacteria.
Extensive and widespread use of antimicrobial drugs led to the emergence of resistant strains of microorganisms. These microorganisms are no longer susceptible to currently available antimicrobial drugs. In order to lower or prevent lethal infectious diseases and maintain public health, new antimicrobial agents are required. This forces researchers to pursue novel antibiotics, not yet resistant by bacteria.
Antimicrobial peptides (AMPs) are part of the armament that insects have developed to fight off pathogens. Although usually cationic, the primary structures of insect AMPs vary markedly. Members of the most frequent AMP families adopt an α-helical conformation in membrane-mimetic environments (Bulet P. et al., Protein and Peptide Letters, 2005, 12, 3-11).
Insects produce antibacterial peptides, which are secreted to their hemolymph, as an innate defense against pathogenic infections (Boman, H. G. et al., Annu. Rev. Microbiol., 1987, 41, 103-126). Some insect species are capable of producing 10-15 different antibiotic peptides (Hoffman, J. A., et al., FEBS Let., 1993, 325, 663-664). Each peptide has a complete different range of antibacterial action (Bulet, P. Medicine Sciences 1999.15, 23-29).
Cecropins were first isolated from the hemolymph of Hyalophora cecropia. Cecropins are small cationic peptides consisting 29-42 amino acid residues, found in the Diptera order (genus Drosophila, Sarcophaga) and Lepidoptera order (genus Hyalophora, Manduca, Bombyx, Antheraea). It should be mentioned that a Cecropin was isolated from porcine intestine (Boman, H. G., et al. Eur. J. Biochem. 1991. 201, 23-31; Morishima, I., et al. Biochem. Physiol. 1990. 95B, 551-554; Steiner, H., et al. Nature 1981. 292, 246-248; Sun, D., et al. Biochem. Biophys. Res. Commun. 1998. 249(2), 410-415; Bulet, P. et al Immunological Reviews. 2004. 198, 169-184). The known sequences for the major Cecropins show that the N-terminal parts are strongly basic while the C-terminal regions are neutral and contain long hydrophobic stretches. In all cases the Cecropins have an amidated C-terminal residue (Boman, H. G. et al., Annu. Rev. Microbiol., 1987, 41, 103-126). Cecropins secondary structure forms two amphiphatic α-helixes which are able to penetrate the bacterial membrane. This ability is followed by membrane loss of ionic gradient balance leading to bacterial death (Christensen, B. C., et al. Proc. Natl. Acad. Sci. USA. 1988 83:1670-1674; Lockey, T. D., et al. Eur. J. Biochem. 1996. 236, 263-271; Marassi, F. M., et al. Biophys. J. 1999. 77, 3152-3155; Wang, W., et al. J. Biol. Chem. 1998. 273, (42) 27438-27448).
Cecropins are very similar molecules as half the amino acid substitutions are strictly conservative. Theoretical predictions and circular dichroism spectra indicate that these peptides can form nearly perfect amphipathic α-helices with charged groups on one longitudinal side and hydrophobic side residues on the opposite side. Proteins with amphipathic helices are often associated with membranes, and this secondary structure may be of importance for the membrane-disrupting activity of the Cecropins (Boman, H. G. et al., Annu. Rev. Microbiol., 1987, 41, 103-126).
The structure of different sequences of peptides of the Cecropin family shows that they represent similar types of molecules. In addition to strongly basic N-terminal region and a long hydrophobic stretch in the C-terminal half, there are other typical conserved features such as: tryptophan at position 2, the single and double lysines at positions 5, 8 and 9 and arginine at position 12. It can be concluded that there must have been strong selection pressures that have conserved certain Cecropin sequences in different types of insects throughout evolution (Boman, H. G., et al. Eur. J. Biochem. 1991. 201, 23-31).